If you want to know the real price of a diamond, try selling it back to a jeweler. You buy it for $4K they might buy it back at $100 if they do it at all.
Jeweler friend-in a high end store- told me years ago that the markup on diamonds is huge. Not only that- jewelry full of tiny diamonds they advertise as ‘total weight ‘ are where they make their money . When big stones are purchased BAGS of tiny point diamonds are given FREE. So all the jewelry with tiny stones is basically worthless except for metal price. Think about that the next time you see ‘-carat total weight’ on a piece.